NASA SAYS THREE FEET OF SEA LEVEL RISE IS UNAVOIDABLE...

Waves batter the seawall at Wimereux, on the Channel coasts of the United Kingdom and northern France. (Photo: Philippe Huguen/AFP/Getty Images)

Scientists say coastal cities around the world are all but certain to face catastrophic flooding in the coming decades.

Ocean levels around the world have risen about three inches since 1992 thanks to warming temperatures owing to the burning of fossil fuels. Now, new NASA research shows sea levels will likely rise three feet in the coming decades, and it’s too late to do anything about it.

The findings are based on satellite data that looked at sea levels, the amount of heat that’s already stored in the oceans, and how much water is being added by melting ice sheets in the Arctic and Antarctica.

“It’s pretty certain we are locked into at least three feet of sea-level rise, and probably more,” Steve Nerem, lead researcher on NASA’s Sea Level Change Team, said on a conference call Wednesday. “But we don’t know whether it will happen within a century or somewhat longer.”

The three-foot figure is at the high end of what the United Nations’ Intergovernmental Panel on Climate Change predicted two years ago, when it estimated one to three feet of sea-level rise by 2100.

Even if the high point of sea-level rise is still a century out, the effects of encroaching seas will be felt much sooner. Low-lying areas such as Bangladesh’s capital, Dhaka, with a population of 14 million, could experience regular river floods within the next two decades. That would leave portions of the city completely inundated. For New Orleans, a city still picking up the pieces from Hurricane Katrina, NASA’s predictions raise concerns about whether projects to protect the city from future floods underestimate sea-level rise.

If the new normal means three feet of sea-level rise, hundreds of millions of people living in coastal regions will feel the effects first as increasingly destructive storms—like Sandy and Katrina—trigger frequent floods that could wipe out millions of homes and billions of dollars’ worth of infrastructure.

Oceans have risen steadily on the East Coast of the United States, but on the West Coast, they have dropped about three inches over the past 20 years. Climate scientists attribute the phenomenon to a weather pattern called the Pacific Decadal Oscillation.

“But this is a temporary thing,” Josh Willis, an oceanographer at NASA’s Jet Propulsion Laboratory, said during the conference call. “We’re going to see sea-level rise on the West Coast sometime over the next 20 years, and we’ll probably see faster-than-average sea-level rise, so we have to be prepared.”

Temperature increases account for at least one-third of the world’s sea-level rise. The oceans, which cover 70 percent of the Earth’s surface, absorb much of that heat.

“When heat goes under the ocean, it expands just like mercury in a thermometer,” Nerem said during the briefing. That expansion means more mass for the ocean and a higher sea level overall.

Rising temperatures from the burning of fossil fuels are also melting the polar ice sheets at an accelerating rate, which contributes to most of the sea-level increase.

This article has been revised to reflect the following correction:

Correction Aug. 28, 2015: An earlier version of this article misstated the time frame for how long scientists estimate it will take sea levels to rise three feet.

Aug 27, 2015

Taylor Hill is an associate editor at TakePart covering environment and wildlife.